Contralateral influence on recruitment of curarized muscle fibres during maximal voluntary extension of the legs.

The force of maximal voluntary contraction (MVC) was compared during extension of one leg alone and during simultaneous extension of both legs. In 6 subjects MVC of two legs extension was 75 +/- 3.6 (S.E.)% of the sum of the single one leg MVC. This may indicate a reduced muscle fibre involvement during two leg extension as compared with one leg extension. We investigated the possibility that either red (type I) or white (type II) muscle fibre recruitment was restricted during two leg extension. The neuromuscular transmission in the two types of fibres was partially blocked with d-tubocurarine (dtc) or decamethonium (C10) for either type I or II, respectively. In 5 subjects receiving dtc the ratio between the reduced two leg and one leg extension forces (75 +/- 2.3%) did not change. During administration of C10, however, this ratio decreased by 16 +/- 3.1%. Partial blocking with dtc caused a two component curve during tension development with a first maximum at 0.22 s, and another maximum at 1.03 s of a 3 s attempt to reach MVC. During partial blocking with C10 results in muscle contractions where a relatively large amount of type I muscle fibres are contributing to the force developed. The experiment suggests that type I fibre recruitment is restricted during severe static exercise in normal muscles. Furthermore, it demonstrates that fibre recruitment can vary in the same muscle function performed with one leg or two legs.